(1) Field of the Invention
The invention relates generally to semiconductors, and more specifically to a method of improving metal lithography techniques and via-plug filling. The method employs planarization and etch-back at judiciously chosen process steps prior to interlevel metal depositions. At the same time, the integrity of the via-plugs is improved by depositing thick, high quality oxide in the deep pocket regions. Use of silicon nitride along with the oxide also reduces the microloading effect.
(2) Description of the Related Art
Further advances in submicron integrated circuit technology demand well defined wafer topography and extremely flat surfaces. The depth-of-field limitations of submicron optical-lithography tools require surfaces to be planar within .+-.0.5 micrometers (.mu.m) as stated in a chapter on "The Need for Planarity," in the book "SILICON PROCESSING for the VLSI Era," by S. Wolf, et al, published by Lattice Press, Sunset Beach, Calif., p. 65. Thus, if surface conditions are such that the nonplanar irregularities--sometimes in the form of steps--on a wafer layer are larger than 0.5 .mu.m, then a flattening or planarization of those surfaces is required.
As is well known in the art of semiconductor fabrication in general, integrated circuits are built on a silicon substrate which is polished and very flat initially. Then isolated active-device regions are created within the single-crystal structure, a process which is benign to causing any surface irregularities, per se. All the subsequent processes, however--such as the opening up of holes to contact the devices, interconnecting the devices by means of metal conductors, and the insulation of those conductors from each other both laterally in a layer (intra-layer), or vertically between levels (inter-level)--until the completion of the total integrated circuit chip, all contribute, in some way or another, to different types of surface irregularities. In prior art, the effects-mechanical, electrical or both--of such irregularities on neighboring components could be minimized by simply spacing out the intra-layer components themselves, or by covering them thickly enough so as to diminish their influence between components in different levels of the semiconductor structure. However, with the advent of submicron technology, space is premium, and therefore, other solutions must be found. Furthermore, as devices themselves become smaller, and as they are packed together ever so closely, some of the other second or third order effects that could be hidden in prior art, now have come forward. For example, a flattening or planarizing process of an interlevel insulator effects the integrity of the interstitial metal that connects one level to another. On the other hand, the process of minimizing the irregularities of a metal layer effects the integrity of the surrounding insulating material. Solutions to these cross-effects are discussed in this invention. Most importantly, a submicron methodology for improving lithographic topography in depositing metal layers is presented.
There are examples, in the prior art, where either one or other planarization method is applied to either the metal layers or the interlevel insulators to solve a particular problem relevant to each, respectively. In this invention, the disclosed solution for one resolves a problem in the other, as well, thusly: specifically, a planarization process comprising of specific materials and procedures is introduced prior to the next level of metallization such that the surface onto which the metal is to be deposited later is flat, smooth, i.e., free of nonplanarities. In the absence of such surface irregularities, then, the imaging surface is ready to accept masks with submicron sized features and is able to accommodate the very small depth of focus of the optical steppers used to pattern the circuits on the wafer. Before the metal is deposited to form the intricate circuitry, first the connections from the level below to the next level must be formed by opening holes in the intervening layer of insulator, or the dielectric, and then filled with metal. The holes in the dielectric are called "vias". The proper way of filling these vias, as a function of the metal and dielectrics used, is one of the concerns of the workers in the field. As will be seen in this invention, the particular, and integrated way of surface planarization disclosed here also alleviates one of the serious via filling problems, namely, "exploding vias."
Vias in the dielectric explode when they are exposed to processing temperatures. This is caused by the expansion and release of gasses (outgassing), from the dielectric through the walls of the vias at the time of the metal deposition into the vias. The phenomenon of exploding vias is particularly observed when the dielectric spin-on-glass (SOG) is integrated with the chemical vapor deposition (CVD) of tungsten (W), or CVD-W. According to S. Wolf in his book "Silicon Processing for the VLSI Era" published by Lattice Press , Sunset Beach , Calif., p 236, the source of the gasses has been found to be primarily moisture that remains within the SOG due to insufficient curing or that has been reabsorbed during the wet process steps performed following spin-on and curing of the SOG film (e.g., during resist removal after the via etch or during pre-metal-deposition cleaning steps.) An additional source for these detrimental gasses is the chemical reduction of tungsten hexafluoride (WF.sub.6) by silicon, hydrogen or silane which yield vapors of the type silicon fluoride (SiF.sub.4), hydrogen fluoride (HF), and SiF.sub.4, respectively. The silane reduction process is preferred. However, when used alone, both the CVD-W process and the use of SOG compare favorably with other approaches. SOG has good shelf life, a simple cure cycle, excellent thermal stability, low stress and good crack resistance, spin-on uniformity, good adhesion and an appropriate film-thickens range. CVD-W exhibits excellent resistance to electromigration effects, hillock formation, and humidity induced corrosion, and other characteristics which readily lend themselves to very high circuit packing densities. Hence, when combined, an integrated SOG/CVD-W process would result in added advantages. The following is a comparative description of prior art and the disclosed method where the latter, by a judiciously chosen process step, integrates SOG with CVD-W resulting in improved metal lithography techniques and via-plug filling.
The alphanumerically designated FIGS. 1(A)-9(A) and 1(B)-9(B) show, respectively, a series of steps involved in depositing various layers on a semiconductor substrate by utilizing prior art and the method of this invention. Altogether, nine figures are shown, and it will be evident in the ensuing discussion that the number of the figures also correspond to the step number of the process depicted in that particular figure, while the alpha-suffix (A) refers to prior art, and (B) to the present invention. To begin, a silicon wafer is provided up to the first level of metal. That is, the various components such as the drain-gate-source have been implanted , isolated, and covered with a dielectric material prior to metallization. In both cases, the first level dielectric is a boron and phosphorous doped BP-ETOS film. Undoped films are deposited by the decomposition of tetraethyl orthosilicate (TEOS). The first metal, which is aluminum copper, AlCu, as shown in the figures, is then deposited by well-known techniques which are not described in detail here in order not to unnecessarily obscure the present invention.
In FIGS. 1(A) and 1(B), the deposition of the first metal is designated as the first step of a series of nine process steps thereafter. All of the steps are applicable to both the prior art and the present invention; with one exception, however; Step 2 of FIG. 2(A), which is introduced in this invention is lacking in the prior art, as seen in comparing FIG. 2(A) with FIG. 2(B). This Step 2 consists of a planarization process judiciously introduced at this juncture so as to shape in the flatness of the substrate at early stages of the semiconductor structure. It will be appreciated that this step could in fact be introduced still earlier at the time of the poly-metal deposition in forming the local interconnects for the source-gate-drain regions. Nevertheless, the positive effects of planarization even at this stage is clearly demonstrated after the next step of metal photo/etch as seen in FIG. 3(B). In the absence of Step 2, and, therefore, at Step 3, the tops of the etched metal in the prior art, FIG. 3(A), are formed at different depths because of the unevenness of the blanket metal in the first place. This then creates different depths of foci for the next level of metal, which is a problem for the optical exposure tools. The net result is then a reduced resolution capability for the optical system which in turn presents a major limiting factor in the processing of submicron feature sized integrated circuits. In the present invention, prior to photo metal/etch in Step 3 of FIG. 3(B), a judicious planarizing is performed to avoid the problem of unevenness and hence optical aberrations in the photolithographic process.
To be able to appreciate the benefits of Step 2, it is well to review briefly the elements of Step 3, namely, that of the photo-metal-etch step in FIG. 3. The patterns that define the circuits on each metal layer in a semiconductor structure are created by lithographic process. That is, layers of photoresist materials are first spin-coated onto the surface to be patterned. Next, the resist layer is selectively exposed through a mask (or recticle in step-and-repeat projection systems) to a form of radiation, such as ultraviolet light, electrons, or x-rays. The mask contains clear and opaque features that define the pattern to be created in the photoresist layer. The patterns in the resist are formed when the photoresist undergoes the subsequent "development" step. The areas of resist remaining after development protect the substrate regions which they cover. Locations from which resist has been removed is then subjected to subtractive etching process in this case, (or additive, in other cases) that transfer the pattern onto the underlying surface.
For the precise transfer of the circuit patterns onto the underlying metal surface to occur, first the transfer of patterns from the mask to the photoresist must be of high resolution. This requires that the depth of focus over an imaging surface be uniform. Though the spin-coated photoresist provides a relatively flat imaging surface for that purpose, it still does not provide the flatness that is now required for resolving submicron-sized features. That is provided with the planarization process of Step 2 in FIG. 2(B) that is disclosed in this invention. Furthermore, it will be observed that when the pattern is finally etched into the metal, the tops of the patterned metal will be formed at elevations where the metal thickness has reached. In the subsequent semiconductor processes, this unevenness in elevation will propagate upwards to upper layers, exacerbating the problem of optical resolution mentioned above. In Step 2 of FIG. 2(B) of this invention, the tops of the metal will be "built-up" and planarized so as to provide the desired flatness for the microlithography that is required in today's submicron technology.
In the critical Step 2 of FIG. 2(B), an initial layer of PECVD-oxide is first deposited using conventional plasma-enhanced chemical vapor deposition techniques. This is to provide an intervening layer between the next spin-on-glass (SOG) layer and the metal, because frequent adhesion failures are observed when SOG is in contact with metal. After being spun on, the liquid SOG is baked to drive off the solvent, and any water that evolves from the film due to the polymerization process. The next process in this planarization step is to etch-back the SOG such that no SOG remains over locations at which vias will be etched. At this point, the surface is micro-planarized, and is ready to accept the appropriate mask for the micro-lithographic process that was described for Step 3 above. After Step 3, a dual layer structure is obtained as shown in FIG. 3(B). Comparing the structures of the prior art with that of the present invention after Step 3, it is now clear that the tops of the latter structure are at the same level. This provides the basis for building up further levels of metallization which are flat, planar and suitable for submicron photolithographic processes. It will be obvious to those skilled in the art, the same planarization described in Step 2 of FIG. 2(B) can be applied at any level of metallization in the processing of semiconductor wafers.
In Steps 4 through 9 (FIGS. 4-9), the integrity of the interconnecting vias is sought. In prior art as shown in FIG. 4(A), PECVD-oxide is deposited over the patterned first level metal, a Step 4. Then, immediately, in Step 5 of FIG. 5(A), SOG is spun over and cured in the conventional way. In Step 6 of FIG. 6(A), the SOG may be partially etched back so that it remains only in the troughs between the metal lines, or not etched back at all so that SOG remains permanently as a continuous thin film between the two layers of PECVD-oxide. The next layer of PECVD-oxide is deposited in Step 7 of FIG. 7(A), and serves as the interlevel dielectric between the two metal layers. Step 8 of FIG. 8(A) is a repeat of Step 3 where mask/photo/etch processes are carried out. The holes so formed in Step 8 are then filled with tungsten to form via-plug interconnections between the first metal layer and the next which is deposited over the plugs as the last process in Step 9 of FIG. 9(A). It is in Step 8 that the vias explode, which this invention teaches how to prevent by judiciously introducing still another process in Step 4.
In Step 4 of FIG. 4(B), the present invention proposes to coat the two-tiered (metal and PECVD-oxide) or dual layer of Step 3 with a thin layer of silicon nitride (SiN). The rest of the Steps 5 through 9 in FIGS. 5 through 9 are the same as described for the prior art above. However, the addition of the SiN layer in Step 4 helps to solve the problem of exploding vias in the following way:
In Step 5 of FIG. 5(B), during baking and plasma ashing of the SOG film, water is evolved from the film due to polymerization of the silanol [SIOH] groups as seen in the reaction
Si--CH.sub.3 +O(plasma)CO.sub.2 +H.sub.2 O+Si--OH PA1 2WF.sub.6 (vapor)+3SiH.sub.4 (vapor)2W(solid)+3SiF.sub.4 (vapor)+6H.sub.2 (vapor)
The loss of considerable mass together with material shrinkage creates a tensile stress in the film although the addition of the organic methyl group CH.sub.3 somewhat resists the concomitant cracking. Should these cracks be unchecked, further outgassing will result from the SOG during Step 6 when CVD-W (tungsten) is deposited in the holes. In the prior art, two detrimental effects are observed. In the silane reduction of the tungsten hexafluoride (WF.sub.6), namely,
the outgassing from the SOG lessens the diffusion of WF.sub.6, thereby causing uneven tungsten coverage in the sidewalls of the hole. The second major problem is the "exploding vias" caused by the expansion and release of gasses in the SOG. In the present invention, both of these problems are avoided by passivating the SOG in the sidewalls by depositing SiN in Step 4 of FIG. 4(B) before Step 5 of FIG. 5(B).
An additional advantage of SiN is that its presence in Step 6 of FIG. 6(B) during SOG etch-back reduces microloading. Briefly, when the etch rate is dependent upon the amount of etchable surface exposed to the etchant, the phenomenon is called a "loading effect." When the etch rate is also sensitive to the pattern density on the wafer surface, as is observed in etching spin-on-glass, it is known as the "micro-loading effect." Loading effects are global and are most commonly encountered in dry-etch processes where they can occur in variety of different conditions. For example, the etch rate can be dependent upon the number of wafers present in an etching chamber. Microloading, on the other hand, is local in the sense that it is affected by the continuously changing etchable area even during the etching process itself. In this invention, the presence of silicon nitride under the SOG layer helps reduce microloading. This is because when the nitride film is etched during the SOG etchback, there is no oxygen released to enhance the etch rate of the SOG film. Consequently, silicon nitride under the SOG layer helps to reduce the microloading effect.
Hence an integrated process is disclosed where the desirable properties of SOG (good planarization) and CVD-W (good deposition) are utilized to yield improved metal lithography and via-plug integrity while the undesirable properties such as outgassing from the SOG are kept under control by judicious choice of process Steps.